This invention relates to the elimination of echoes in communication signal paths and, more particularly, to effective cancellation of echoes by use of an accurate and highly efficient model of the echo path transfer function.
Since echoes in telephone circuits have a disturbing influence on conversation, a number of techniques have been devised to mitigate their effect. Echo suppression was the first technique to be contrived. Typically, echo suppression involves some form of selective attenuation automatically operated in response to voice levels in the transmission paths so that the echo that would otherwise be returned to the talker is suppressed. Such arrangements are generally satisfactory for terrestrial communication paths in which the echo delay or the round-trip propagation time between the source of the signal and the return of the echo is not long.
In communication paths via satellite links, the transmission delays are much longer and the echo is more disturbing and disrupts conversation. Echo suppression techniques basically interrupt the return signal path and have a tendency to mutilate speech by chopping the return signal during intervals wherein both parties are talking; i.e., double talking. This degradation of quality of the communication is subjectively more severe when the signals experience long propagation delays in transmission between the parties. Thus, echo suppressors will probably introduce their own signal degradation in the process of eliminating echoes.
Another more sophisticated approach utilizes echo cancellation wherein a replica of the echo is automatically constructed or adaptively synthesized from the original signal and subtracted from the return signal to eliminate the echo. Most conventional echo cancellers synthesize the replica using a tapped delay line with adjustable multipliers in an adaptive feedforward arrangement also called a transversal filter. The multipliers are automatically adjusted by a control signal derived from the difference between the echo and the replica. Since the impulse response of an echo path may be rather long, accurate synthesis of the replica by transversal filters to effect echo cancellation may require many taps and associated multipliers, an arrangement which is complex and costly. In fact, echo cancellers have not been generally utilized to any great extent because of their high cost.
Feedback or recursive arrangements which have an inherently long impulse response appear to be able to synthesize the replica accurately. Since recursive arrangements are simpler, their use would seem to provide a reduction in complexity and a corresponding lower cost in achieving echo cancellation. However, an inherent difficulty with the recursive arrangement is that its operation cannot be readily adapted by automatic control in order to minimize the mean-squared residual echo. In a practical application, the recursive circuit will not likely converge to the operating point that will provide the most effective echo cancellation because characteristically there are several sub-optimum multiplier tap settings to which the adaptation algorithm can converge rather than an optimum unique minimum as is the case for the conventional feedforward echo canceller.